1. Technical Field
Embodiments of the present invention generally relate to a sheet thickness detector to detect the thickness of a sheet to be supplied, and an image forming apparatus incorporating the sheet thickness detector.
2. Related Art
In image forming apparatuses such as printers, copiers, and facsimile machines forming an image on a sheet of recording medium, image forming conditions are optimized according to sheet thickness for producing a high-quality image.
However, such optimization includes complicated and/or costly configurations, and provides uneven detection results.
In a transfer process for transferring toner to the recording medium, a volume resistance varies depending on a thickness of a sheet. Therefore, a transfer current to drive a transfer charger needs to be changed according to the thickness of a sheet. Further, in a fixing process for fixing toner on a sheet to the sheet by application of heat and pressure, the appropriate quantity of heat is different according to the thickness of a sheet. Therefore, the temperature changes according to the thickness of the sheet.
A sheet thickness detector of an example includes a reference roller, a detection roller, and a detection lever. The detection lever has one end that is attached to the detection roller to detect an amount of displacement of a surface of the detection roller and the other end that is a free end to move in a direction that the detection roller separates from the reference roller, that is, a direction of thickness of a sheet and in an axial direction of the reference roller.
The detection roller in the sheet thickness detector of the present example has a rotary shaft that has a length greater than the entire lateral length of a sheet in a direction perpendicular to the sheet conveyance direction, which is the entire width thereof. Since the detection roller is rotated about the rotary shaft in the sheet conveyance direction, detection of an amount of displacement with respect to the rotary shaft or surface of the detection roller indicates the amount of displacement including disposition or eccentricity of the rotary shaft. Therefore, the amount of displacement by an amount equivalent to the thickness of the sheet may not be detected accurately.
In a sheet thickness detector of another example, the diameter of a part of at least one of a reference roller and a detection roller is reduced. A displacement member that is displaced according to the passage of a sheet of recording medium is arranged at the part of the reduced diameter while being engaged with one of the reference roller and the detection roller. With this configuration of the second example, the thickness of the sheet is detected based on the amount of displacement of the displacement member.
Even though not having a configuration that directly detects the amount of displacement of the detection roller, the sheet thickness detector of this example has a configuration that detects an amount of displacement of a displacement member operating together with the detection roller, and therefore is negatively affected by rotational fluctuation of the detection roller. Further, this configuration is so complicated to install in a compact image forming apparatus, which is likely to increase its manufacturing cost.
Similarly, in a sheet thickness detector of yet another example, the diameter of a part of at least one of a reference roller and a detection roller is reduced. However, a displacement member that is displaced according to the passage of a sheet of recording medium is arranged at the part of the reduced diameter while being separated from the reference roller and the detection roller. With this configuration of the second example, the thickness of the sheet is detected based on the amount of displacement of the displacement member.
The sheet thickness detector of this example in which the detection roller and the displacement member operate separately is expected to avoid the negative effect due to the rotational fluctuation of the detection roller. However, the complicated configuration of the displacement member makes it difficult to provide the displacement member in a space-saving device or apparatus such as an image forming apparatus, which is also likely to increase the cost.